Typically in a headphone, a driver unit disposed within a housing drives a diaphragm according to an audio signal to thereby vibrate air to generate sound. Here, it is known that an acoustic characteristic of the headphone depends on a structure within the housing. Specifically, the acoustic characteristic of the headphone can vary according to the volume of a space provided within the housing, a size of a ventilation hole, which can be a passage of air, formed within the housing, or the like. Therefore, a number of techniques on the structure within the housing have been proposed.
There has been disclosed a sealed-type canal-type earphone in which a space spatially blocked from the outside, except for an opening for outputting sound toward the outside, is formed between a housing and a front face side as a side provided with a diaphragm of a driver unit (see, for example, Patent Literature 1). Furthermore, there has been disclosed a technique for improving an acoustic characteristic by providing a tubular duct unit, which spatially connects between the inside and the outside of a housing, on a rear side of the housing as a side opposite to the side provided with a diaphragm of a driver unit (see, for example, Patent Literature 2).